That cut/repair looks well dodgey. I used a seat belt manufacturer around Crawley area sometime ago to extend some seatbelts on a classic car to enhance tge safety for rear seat passengers. All done to proper standards. I shall take a look and see if I can find a link.
I would disconnect the battery and leave for 30 minutes before touching it. It may be alright but a company called airbag reset or something like that can always interrogate the system and sort it for you.
I have used these before and use them again if I needed seatbelt repairs.
Give them a ring or an email explaining your problem and see what they say, good luck.